The amount of additional future temperature change following a complete
cessation of CO2 emissions is a measure of the unrealized warming to
which we are committed due to CO2 already emitted to the ...atmosphere.
This “zero emissions commitment” (ZEC) is also an important quantity when
estimating the remaining carbon budget – a limit on the total amount of
CO2 emissions consistent with limiting global mean temperature at a
particular level. In the recent IPCC Special Report on Global Warming of
1.5 ∘C, the carbon budget framework used to calculate the
remaining carbon budget for 1.5 ∘C included the assumption that
the ZEC due to CO2 emissions is negligible and close to zero. Previous
research has shown significant uncertainty even in the sign of the ZEC. To
close this knowledge gap, we propose the Zero Emissions Commitment Model
Intercomparison Project (ZECMIP), which will quantify the amount of
unrealized temperature change that occurs after CO2 emissions cease and
investigate the geophysical drivers behind this climate response.
Quantitative information on ZEC is a key gap in our knowledge, and one that
will not be addressed by currently planned CMIP6 simulations, yet it is
crucial for verifying whether carbon budgets need to be adjusted to account
for any unrealized temperature change resulting from past CO2
emissions. We request only one top-priority simulation from comprehensive
general circulation Earth system models (ESMs) and Earth system models of
intermediate complexity (EMICs) – a branch from the 1 % CO2 run with
CO2 emissions set to zero at the point of 1000 PgC of total CO2
emissions in the simulation – with the possibility for additional
simulations, if resources allow. ZECMIP is part of CMIP6, under joint
sponsorship by C4MIP and CDRMIP, with associated experiment names to enable
data submissions to the Earth System Grid Federation. All data will be published
and made freely available.
SignificanceRecord-setting fires in the western United States over the last decade caused severe air pollution, loss of human life, and property damage. Enhanced drought and increased biomass in a ...warmer climate may fuel larger and more frequent wildfires in the coming decades. Applying an empirical statistical model to fires projected by Earth System Models including climate-ecosystem-socioeconomic interactions, we show that fine particulate pollution over the US Pacific Northwest could double to triple during late summer to fall by the late 21st century under intermediate- and low-mitigation scenarios. The historic fires and resulting pollution extremes of 2017-2020 could occur every 3 to 5 y under 21st-century climate change, posing challenges for air quality management and threatening public health.
The observed global net land carbon sink is captured by current land models. All models agree that atmospheric CO2 and nitrogen deposition driven gains in carbon stocks are partially offset by ...climate and land-use and land-cover change (LULCC) losses. However, there is a lack of consensus in the partitioning of the sink between vegetation and soil, where models do not even agree on the direction of change in carbon stocks over the past 60 years. This uncertainty is driven by plant productivity, allocation, and turnover response to atmospheric CO2 (and to a smaller extent to LULCC), and the response of soil to LULCC (and to a lesser extent climate). Overall, differences in turnover explain ~70% of model spread in both vegetation and soil carbon changes. Further analysis of internal plant and soil (individual pools) cycling is needed to reduce uncertainty in the controlling processes behind the global land carbon sink.
This study assesses the impacts of stratospheric aerosol intervention (SAI) and solar dimming on stratospheric ozone based on the G6 Geoengineering Model Intercomparison Project (GeoMIP) experiments, ...called G6sulfur and G6solar. For G6sulfur, an enhanced stratospheric sulfate aerosol burden reflects some of the incoming solar radiation back into space to cool the surface climate, while for G6solar, the reduction in the global solar constant in the model achieves the same goal. Both experiments use the high emissions scenario of SSP5-8.5 as the baseline experiment and define surface temperature from the medium emission scenario of SSP2-4.5 as the target. In total, six Earth system models (ESMs) performed these experiments, and three out of the six models include interactive stratospheric chemistry. The increase in absorbing sulfate aerosols in the stratosphere results in a heating of the lower tropical stratospheric temperatures by between 5 to 13 K for the six different ESMs, leading to changes in stratospheric transport, water vapor, and other related changes. The increase in the aerosol burden also increases aerosol surface area density, which is important for heterogeneous chemical reactions. The resulting changes in the springtime Antarctic ozone between the G6sulfur and SSP5-8.5, based on the three models with interactive chemistry, include an initial reduction in total column ozone (TCO) of 10 DU (ranging between 0–30 DU for the three models) and up to 20 DU (between 10–40 DU) by the end of the century. The relatively small reduction in TCO for the multi-model mean in the first 2 decades results from variations in the required sulfur injections in the models and differences in the complexity of the chemistry schemes. In contrast, in the Northern Hemisphere (NH) high latitudes, no significant changes can be identified due to the large natural variability in the models, with little change in TCO by the end of the century. However, all three models with interactive chemistry consistently simulate an increase in TCO in the NH mid-latitudes up to 20 DU, compared to SSP5-8.5, in addition to the 20 DU increase resulting from increasing greenhouse gases between SSP2-4.5 and SSP5-8.5. In contrast to G6sulfur, G6solar does not significantly change stratospheric temperatures compared to the baseline simulation. Solar dimming results in little change in TCO compared to SSP5-8.5. Only in the tropics does G6solar result in an increase of TCO of up to 8 DU, compared to SSP2-4.5, which may counteract the projected reduction in SSP5-8.5. This work identifies differences in the response of SAI and solar dimming on ozone for three ESMs with interactive chemistry, which are partly due to the differences and shortcomings in the complexity of aerosol microphysics, chemistry, and the description of ozone photolysis. It also identifies that solar dimming, if viewed as an analog to SAI using a predominantly scattering aerosol, would succeed in reducing tropospheric and surface temperatures, but any stratospheric changes due to the high forcing greenhouse gas scenario, including the potential harmful increase in TCO beyond historical values, would prevail.
We present the latest version of the ISBA‐CTRIP land surface system, focusing on the representation of the land carbon cycle. We review the main improvements since the year 2012, mainly added modules ...for wild fires, carbon leaching through soil and transport of dissolved organic carbon to the ocean, and land cover changes but also improved representation of photosynthesis, respiration, and plant functional types. This version of ISBA‐CTRIP is fully described in terms of land carbon pools, fluxes, and their interactions. Results are compared with the previous version in an off‐line mode forced by observed climate during the historical time period. The two simulations are presented to demonstrate the model performance compared to an ensemble of observed and observation‐derived data sets for gross and net primary productivity, heterotrophic and autotrophic respiration, above and below ground biomass, litter, and soil carbon pools. New developments specific to the new version such as burned area, fire emissions, carbon leaching, and land cover are also validated against observations. The results show clearly that the latest version of ISBA‐CTRIP outperforms the former version and reproduces generally well the observed mean spatial patterns in carbon pools and fluxes, as well as the seasonal cycle of leaf area index. The trends of the global fluxes over the last 50 years agree with other global models and with available estimates. This comparison gives us confidence that the model represents the main processes involved in the terrestrial carbon cycle and can be used to explore future global change projections.
Plain Language Summary
The land surface exchanges energy, water, and carbon with the atmosphere and partly controls the atmospheric CO2 concentration. It is therefore crucial to represent correctly the carbon cycle on land in models designed to be used in Earth System Models. We present here the improvements made to the representation of the land carbon cycle by the land surface system ISBA‐CTRIP. We improved the representation of several processes using published data, and we added processes that were not represented. The new version of the model performs better than the previous one at representing the carbon fluxes and pools, when compared to a series of observation data sets. This evaluation suggests that we can use ISBA‐CTRIP to explore the changing climate and carbon cycle.
Key Points
This paper documents the updates to the biogeochemical module of the ISBA‐CTRIP land surface system for use in the CNRM‐ESM 2‐1 Earth system model
The newly represented processes are the leaching of carbon and transport of dissolved organic carbon to the ocean, fire with area burned and carbon emissions, and land cover changes
The largest improvements in the representation of net primary productivity are due to improved autotrophic respiration
Many nations responded to the COVID-19 pandemic by restricting travel and other activities during 2020, resulting in temporarily reduced emissions of CO2, other greenhouse gases and ozone and aerosol ...precursors. We present the initial results from a coordinated Intercomparison, CovidMIP, of Earth system model simulations which assess the impact on climate of these emissions reductions. Twelve models performed multiple initial-condition ensembles to produce over 300 simulations spanning both initial condition and model structural uncertainty. We find model consensus on reduced aerosol amounts (particularly over southern and eastern Asia) and associated increases in surface shortwave radiation levels. However, any impact on near-surface temperature or rainfall during 2020-2024 is extremely small and is not detectable in this initial analysis. Regional analyses on a finer scale, and closer attention to extremes (especially linked to changes in atmospheric composition and air quality) are required to test the impact of COVID- 19-related emission reductions on near-term climate.
To limit global warming to well below 2 ° most of the IPCC-WGIII future stringent mitigation pathways feature a massive global-scale deployment of negative emissions technologies (NETs) before the ...end of the century. The global-scale deployment of NETs like Biomass Energy with Carbon Capture and Storage (BECCS) can be hampered by climate constraints that are not taken into account by Integrated assessment models (IAMs) used to produce those pathways. Among the various climate constraints, water scarcity appears as a potential bottleneck for future land-based mitigation strategies and remains largely unexplored. Here, we assess climate constraints relative to water scarcity in response to the global deployment of BECCS. To this end, we confront results from an Earth system model (ESM) and an IAM under an array of 25 stringent mitigation pathways. These pathways are compatible with the Paris Agreement long-term temperature goal and with cumulative carbon emissions ranging from 230 Pg C and 300 Pg C from January 1st onwards. We show that all stylized mitigation pathways studied in this work limit warming below 2 °C or even 1.5 °C by 2100 but all exhibit a temperature overshoot exceeding 2 °C after 2050. According to the IAM, a subset of 17 emission pathways are feasible when evaluated in terms of socio-economic and technological constraints. The ESM however shows that water scarcity would limit the deployment of BECCS in all the mitigation pathways assessed in this work. Our findings suggest that the evolution of the water resources under climate change can exert a significant constraint on BECCS deployment before 2050. In 2100, the BECCS water needs could represent more than 30% of the total precipitation in several regions like Europe or Asia.
Solar radiation management by stratospheric aerosol injection (SRM‐SAI) has been proposed as a possible method to counteract anthropogenic global warming, with climate models suggesting it could ...reduce substantially global temperature and associated impacts. Its effectiveness as simulated by Earth system models exhibits, however, large uncertainties, implying high risks for natural and human ecosystems. Here we identify an emergent relationship linking the long‐term global land surface cooling due to SRM‐SAI and the short‐term cooling following the twentieth century major volcanic eruptions across an Earth system models ensemble. This emergent relationship, combined with observations and reanalysis data, is used to constrain the global land surface temperature (LT) response to reduced downward solar radiation. Based on these constraints, we find a mean decrease in land surface temperature of 0.44 K·W−1·m2, 20% smaller than the unconstrained multimodel mean. This new estimate may affect how trade‐offs between cost, risk, and effectiveness of SRM‐SAI might be considered.
Plain Language Summary
Solar radiation management by stratospheric sulfate aerosol injection has been proposed 10 years ago as means of limiting the harmful impacts of anthropogenic global warming in the absence of strong global efforts toward mitigation of greenhouse gas emissions. However, in absence of real‐world experiments, the efficiency of this method to counteract global warming as derived from model simulated remains poorly constrained. Here using state‐of‐the‐art multimodel simulations, we have identified an emergent property of the Earth system between the decrease in solar radiation and the global land surface temperature. First, we relate long‐term climate sensitivity to the short‐term response of a natural observable analog in large volcanic eruptions. When applied to multiple data sets, which relate the observed temperature decrease to changes in incoming shortwave radiation, we constrain the model response to sulfate geoengineering. We find that state‐of‐the‐art multimodel simulations overestimate the efficiency of this method to counteract global warming by about 20%.
Key Points
A robust intermodel relationship emerges between the land surface cooling associated with SRM and the cooling following volcanic eruptions
This emergent relationship suggests that CMIP5 models overestimate the climate response to SRM by ~20% with regard to available observations
The uncertainty in modeled climate response to SRM is reduced by ~30% using of this emergent constraint approach
In order to explore the effects of mesoscale eddies on marine biogeochemistry over climate timescales, global ocean biogeochemical general circulation models (OBGCMs) need at least to be run at a ...horizontal resolution of a 0.25°, the minimal resolution admitting eddies. However, their use is currently limited because of a prohibitive computational cost and storage requirements. To overcome this problem, an online coarsening algorithm is evaluated in the oceanic component (NEMO‐GELATO‐PISCES) of CNRM‐ESM2‐1. This algorithm allows to compute biogeochemical processes at a coarse resolution (0.75°) while inheriting most of the dynamical characteristics of the eddy‐admitting OBGCM (0.25°). Through the coarse‐graining process, the effective resolution of the ocean dynamics seen by the biogeochemical model is higher than that which would be obtained from an OBGCM run at 0.75°. In this context, we assess how much the increase from low (1°) to coarse‐grained horizontal resolution impacts the ocean dynamics and the marine biogeochemistry over long‐term climate simulations. The online coarsening reduces the computational cost by 60% with respect to that of the eddy‐admitting OBGCM. In addition, it improves the representation of chlorophyll, nutrients, oxygen, and sea‐air carbon fluxes over more than half of the open ocean area compared to the 1° OBGCM. Most importantly, the coarse‐grained OBGCM captures the physical‐biogeochemical coupling between sea‐air carbon fluxes and sea surface height and between oxygen minimum zone boundaries and eddies, as produced by the eddy‐admitting OBGCM. Such a cost‐efficient coarsening algorithm offers a good trade‐off to conduct process‐based studies over centennial timescales at higher resolution.
Key Points
The eddy‐admitting (0.25°) model replicates observed physical‐biogeochemical coupling, whereas the 1° horizontal resolution model does not
The computation cost of the eddy‐admitting model is divided by 2.7 when using a coarse‐grained grid for marine biogeochemistry
The coarse‐grained solution inherits the key features of the 0.25° solution, including the physical‐biogeochemical coupling
The impact of anthropogenic climate change on marine net primary production (NPP) is a reason for concern because changing NPP will have widespread consequences for marine ecosystems and their ...associated services. Projections by the current generation of Earth system models have suggested decreases in global NPP in response to future climate change, albeit with very large uncertainties. Here, we make use of two versions of the Institut Pierre-Simon Laplace Climate Model (IPSL-CM) that simulate divergent NPP responses to similar high-emission scenarios in the 21st century and identify nitrogen fixation as the main driver of these divergent NPP responses. Differences in the way N fixation is parameterised in the marine biogeochemical component PISCES (Pelagic Interactions Scheme for Carbon and Ecosystem Studies) of the IPSL-CM versions lead to N-fixation rates that are either stable or double over the course of the 21st century, resulting in decreasing or increasing global NPP, respectively. An evaluation of these two model versions does not help constrain future NPP projection uncertainties. However, the use of a more comprehensive version of PISCES, with variable nitrogen-to-phosphorus ratios as well as a revised parameterisation of the temperature sensitivity of N fixation, suggests only moderate changes in globally averaged N fixation in the 21st century. This leads to decreasing global NPP, in line with the model-mean changes of a recent multi-model intercomparison. Lastly, despite contrasting trends in NPP, all our model versions simulate similar and significant reductions in planktonic biomass. This suggests that projected plankton biomass may be a more robust indicator than NPP of the potential impact of anthropogenic climate change on marine ecosystems across models.
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